GCN Circular 13844

S. D. Barthelmy (GSFC), S. T. Holland (STScI) and J. A. Kennea (PSU)
report on behalf of the Swift Team:

At 02:54:21 UT, the Swift Burst Alert Telescope (BAT) triggered and
located an outburst from SGR 1806-20 (trigger=535747).  Swift slewed
immediately to the source.  The BAT on-board calculated location is 
RA, Dec 272.130, -20.410 which is 
   RA(J2000)  =  18h 08m 31s
   Dec(J2000) = -20d 24' 35"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single spike
with a duration of about 0.064 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~0.0 sec after the trigger. 

The XRT began observing the field at 02:55:19.9 UT, 58.6 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 272.1635, -20.4112 which is
equivalent to:
   RA(J2000)  = 18h 08m 39.24s
   Dec(J2000) = -20d 24' 40.3"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 113 arcseconds from the BAT onboard position, within the
BAT error circle. This position may be improved as more data are
received; the latest position is available at
http://www.swift.ac.uk/sper. This position is 1.4 arcseconds from that
of SGR 1806-20. 

A power-law fit to a spectrum formed from promptly downlinked event
data does not constrain the column density. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 63 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT error circle. The list of sources is typically
complete to about 18 mag. No correction has been made for the large, but
uncertain extinction expected.